1. Field of the Invention
The present invention generally relates to nonvolatile semiconductor memory devices, and particularly relates to a nonvolatile semiconductor memory device that is programmable.
2. Description of the Related Art
In a nonvolatile semiconductor memory device that is programmable and performs erasure on a batch basis, all the data in a sector to be erased are first set to “0” by preprogramming, and, then, the sector is erased. After the erasure, over-erasure correction is performed to correct memory cells that were over-erased within the erased sector. In general, thresholds of memory cells are not identical to each other, but vary within some distribution. Memory cells after erasure may include memory cells that end up having negative thresholds because of over-erasure.
Such over-erased memory cells allow an electric current to run through all the time even when the word line is not activated, thereby disturbing read operations and causing malfunction. The over-erasure correction brings the thresholds of such memory cells back to the normal thresholds, thereby eliminating over-erased memory cells.
In the nonvolatile semiconductor memory device as described above, data stored in memory cells of an erased sector may have unknown data status if an erasure operation is halted by a power failure or the like. If there are no over-erased memory cells, an erasure command is entered again into the nonvolatile semiconductor memory device, thereby performing preprogramming, data erasure, and over-erasure correction so as to set the relevant sector in a properly erased state. Depending on the timing of a power failure or resetting during the erasure operation, however, the erasure operation may come to a halt while over-erased memory cells are present.
In such a case, an erasure command may be entered again into the nonvolatile semiconductor memory device, but a sufficient potential cannot be given to a data bus in respect of the over-erased memory cells. This results in a failure to properly write data during the preprogramming stage of the erasure operation. The time limit set for an erasure operation may thus come to an end, resulting in a device hang-up. If this is the case, an erasure operation will always result in a device hang-up no matter how many times erasure operations are performed. Such a nonvolatile semiconductor memory device cannot be used as a memory device any longer.
Accordingly, there is a need for a nonvolatile semiconductor memory device that allows over-erased memory cells to be recovered when the over-erased memory cells are created by the halt of an erasure operation.